Helicopter rescue hoists are used for search and rescue operation of people under emergency situations, for example, evacuation from fire, mid sea, crevasses, land mass, mines and so on under natural calamities or human operational scenarios. In such situations, the hoist needs to be controlled for reel-in/reel-out modes very accurately and reliably, to ensure that a person being rescued is not injured or payload is not damaged. The reliable usage of an electrically powered hoist may be dependent on the efficiency and reliability of the braking systems. The braking mechanism holds the hoist in a fixed position when the motor driving it is not powered. During such operations of a hoist, there could be chances of jerking the payload or even free fall should the braking mechanism become inoperative. In conventional hoist operation, brake function is provided through separate electromechanical means. Some hoist systems have even been equipped with two independent brakes, one of which is known as mechanical load brake and the other a spring set, electrically releasable, friction brake generally referred to as holding brake. A mechanical load brake is a device wherein friction surfaces are brought into engagement by means of torque derived from the suspended load in a manner to retard and stop the descent of said load. The frictional surfaces tend to be released from engagement by torque from the motor in the lowering direction. However, if the motor torque is then removed such as through a power supply failure, the brake would stop and hold the load. The load brake is typically disengaged during hoisting by a clutching mechanism. In operation, each of these brake types are designed to stop and hold any load within the capacity of the hoist. However, use of load and holding brakes is more complex and requires additional weight. The additional weight, in an aircraft application, is undesirable.
While it has long been recognized that a dual braking system where each brake was effective in the event of power failure was a highly desirable feature, several types of hoists have omitted using a mechanical load brake and relied on other schemes for braking when needed. One approach that has been employed is to use electromagnetic motor braking as the second braking means. The phenomenon of electromagnetic braking is a well-known characteristic of motors where they are controlled to operate like a generator, yet also provide a braking torque. On hoists powered from a direct current source, it is quite simple to connect the direct current drive motor such that it becomes a self-excited generator and, thus, provides the second source of braking in the absence of external power. On alternating current powered hoists, while possible, the feature is more complex and as a result, more costly to attain. Finally, it should be noted that electromagnetic braking in any form, since it depends on rotation to develop torque, would not always hold a suspended load stationary. As a result, it at best provides controlled lowering of the load, fortunately at a speed often below the normal lowering speed, and therefore can be advantageous, especially compared to an uncontrolled free falling of a hoist load. Therefore, it would be desirable to have an alternative means of providing braking for the hoist that takes advantage of electromagnetically braking of the hoist motor, resulting in faster and more accurately controlled stopping action, particularly should the conventional electromagnetic brake become inoperative.